Electronic – Why does GSM cause speakers to buzz


Based on numerous internet resources, speaker wire acts like an antenna which picks up the transmitted signal of nearby cellphones and causes the speakers to buzz. But I'm not really buying that…

A 3.5 mm speaker cable is designed to carry 1 V. I've seen old setups where PC speakers are powered directly from the 3.5 mm jack (and I've tested playing unamplified sound directly from a PC through the jack, although the volume in my setup was not very high at all). How can the tiny bit of EM emitted by a cellphone radio cause a speaker system, designed to operate off of a fluctuating 1 v signal, produce such a loud buzzing noise? I couldn't imagine the EM generating more than a few micro-volts in a receiving antenna. Am I wrong?


Updated – corrected voltage of line out to 1 V (see comments)

I looked it up, and yes it seems GSM transmits at 2 W. I'd like to do a sanity check with that figure to verify some of the answers which state that the transmitted power is significant. My physics is quite rusty, but I'll try…

We know that the intensity of EM radiation around a source is:

$$I = \frac{P}{4\pi r^2}$$

So let's say we have a wire 2 m long and 0.2 mm wide (I hope this is a valid approximation for the wire) that is approximately 2 m away from a transmitting GSM module.

Then for \$P = 2 W, I = 39 \frac{mW}{m^{2}}\$

Multiply that by the surface area of the wire (0.2 mm * 2 m)

The total EM power along the wire is then 16 \$\mu W\$.

Like I said I'm quite rusty, but is this not correct? Is this really significant enough to produce that sound without being amplified somehow? Perhaps the signal resonates? Or interferes directly with sound cards?

Best Answer

The buzzing is AM detected signal.

The reason of audio amplifiers being hit by GSM signal is that contemporary audio semiconductor parts are actually very functional up to high GHz range. For GSM-800-900MHz range any 80mm copper trace works like 1/4 wave antenna, or stripline resonator. The signal is AM detected on any non-linearity (transistors or diode structures in chips) on multiple points of amplifier simultaneously, also including power regulator chips and so on.

It is translated into audio range as tiny but very sharp and periodic dips or pops of averaged conductivity of non-linear parts (AM detection), which are DC powered.

Think of low speed oscilloscope trace showing straight line with beads of UHF flashes. Simple sharp spikes of consumed DC current will become audible with amplifier.